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platinum-wire being placed in the flask to prevent bumping"*
The flask is surmounted with a fractionating column, in \vhicli
the thermometer is fixed. Various forms of fractionating columns
are used (see Fig, 76).
The effect of the columi%may be explained as follows: the
vapour given off from a mixture of liquids contains a larger pro-
portion of the more volatile constituent than the liquid. If this
vapour is condensed in its ascent, the vapour above this con-
densed liquid will be still richer in the more volatile constituent.
If, by a series of constrictions or diaphragms, the condensed
liquid is obstructed in its return flow, a momentary equilibrium
between liquid and vapour is established at each diaphragm, ancl
the longer the column the greater will be the amount of more
volatile constituent in the last portion of vapour to undergo con-
densation. This passes off by the condenser and is collected
in the receiver. The apparatus (Fig. 76, E) can be made out
of a piece of wide tubing. This is constricted in the blow-pipe
flame, near one end, and a piece of copper wire-gauze with a.
circular hole, carrying the little bent tube, is placed on the con-
striction. A second constriction is made and another gauze
diaphragm introduced. The number of diaphragms may vary
from 10 to 20, according to the degree of separation required.1
Commercial 50 per cent, and 90 per cent. Benzene
are mixtures of benzene and larger or smaller quantities of its
higher boiling homologues, viz., toluene (b. p, 110) and the
xylenes (b. p. 137143). The constituents may be separatedtby
fractional distillation.
Fit up an apparatus with fractionating column and distil
200 c.c. 50 per cent or 90 per cent, benzene, at a regular spfed,
so that the drops falling from the end 'of the condenser may be
readily counted. Collect the distillate between every five degrees
in separate flasks. Redistil each of these fractions in order,
adding the next to the residue of the previous one in the
distilling-flask. Collect portions boiling below 85 and above
105, between every two or three degrees. It will be found that
by a repetition of the process the liquid is gradually separated
into two large fractions, consisting chiefly of benzene and toluene,
and a number of smaller intermediate fractions. The following"
table gives the nolume in c.c., and the- boiling points
0C'* l899) 76, 700,